Circuit boards with semiconductor devices such as flip chips must often be protected from the environment in which the board is employed. One widely-practiced approach is to enclose a circuit board in an assembly composed of a pair of case halves that are secured together with fasteners to form an enclosure that supports and protects the circuit board within. Connectors attached to one of the case halves provide for electrical interconnection to the circuit board. The assembly typically includes a sealing material between the case halves to exclude moisture from the enclosure. The resulting package must then be tested for leakage to ensure the package was properly assembled and sealed.
Because the above-described assembly process is labor intensive, less complicated assembly processes are highly desirable. One solution is an overmolded assembly disclosed in U.S. patent application Ser. No. 09/081,498. The assembly includes a circuit board, a heatsink with pedestals that thermally contact one or more circuit components mounted to the circuit board, and an optional retainer that biases the components into contact with the heatsink pedestals. An overmolded body encases the circuit board and retainer such that, with the heatsink, the overmolded body forms a moisture-impermeable seal around the circuit board and its circuit components. The circuit board, heatsink and retainer define cavities into which the overmold material, such as a thermosetting epoxy, flows during molding to encase the circuit board.
While the above-noted overmolded electronic assembly provides an uncomplicated method for encasing a circuit board and its circuit devices in a protective enclosure, it does not provide or allow for simultaneously underfilling surface-mount devices, such as flip chips and ball grid array (BGA) packages that are physically and electrically connected to the board with numerous solder bump connections. Underfilling is well known for promoting the reliability of flip chips and BGAs on organic circuit boards, and entails completely filling the void between the component and the circuit board with a specially formulated underfill material, such as a thermosetting epoxy. The underfill process has conventionally entailed placement of the underfill material along the perimeter of the component so that capillary action draws the material beneath the component and completely encapsulates the solder bump connections. For optimum reliability, underfill process parameters must be carefully controlled so that voids are not present in the underfill material beneath the component, and to ensure that a uniform fillet is formed along the entire perimeter of the component.
Because of the above process demands, different materials have been employed to underfill surface-mount components and overmold circuit boards. For example, overmolded assemblies in accordance with U.S. patent application Ser. No. 09/081,498 have been manufactured with surface-mount components underfilled with a specially formulated underfill material, after which the circuit board is overmolded with a second and different material specially formulated for this purpose. Another restriction has been the conventional wisdom that, for reliability, the underfill material must not contact the backside of the die or flow between the die and any surface contacting its backside.